Question on re-heating bottles etc.

I do what I know many of you do with Toby's bottles... I make them up with cooled boiled water and leave sealed, on the side, at room temp... when he wants a bottle I add the powder and re-heat in the bottle warmer.

BUT, I suddenly wondered yesterday whether I should be re-heating the water first, THEN adding the powder?!?!?! Have I got it really wrong!?!?!?!?